Gone with the Wind

A recent post at RealClimate by Matthew England discusses the results of his (and others’) recent paper (England et al. 2014, Recent intensification of wind-driven circulation in the Pacific and the ongoing warming hiatus, Nature Climate Change, doi:10.1038/nclimate2106) about changes in wind patterns in the tropical Pacific, their impact on ocean circulation, and the resulting impact on global temperature.

England gives an excellent 1-paragraph summary thus:

A consistent picture has now emerged to explain the slowdown in global average SAT since 2001 compared to the rapid warming of the 1980s and 1990s: this includes the link between hiatus decades and the Interdecadal Pacific Oscillation, the enhanced ocean heat uptake in the Pacific (see previous posts) and the role of East Pacific cooling. All of these factors are consistent with a picture of strengthened trade winds, enhanced heat uptake in the western Pacific thermocline, and cooling in the east – as you can see in this schematic:

All of this serves to emphasize that many of the proposed explanations for the putative “pause” in global surface temperature are far from mutually exclusive. For instance, England notes “there are obvious parallels to Kosaka and Xie’s study assessing the impact of a cooler East Pacific,” and the cooling effect of the el Nino southern oscillation is also implicated in Foster & Rahmstorf (2011).

What struck me most was their clear statement of the results of their study:

The slowdown in warming occurs as a combined result of both increased heat uptake in the Western Pacific Ocean, and increased cooling of the east and central Pacific (the latter leads to atmospheric teleconnections of reduced warming in other locations). We find that the heat content change within the ocean accounts for about half of the slowdown, the remaining half comes from the atmospheric teleconnections from the east Pacific.

This, of course, made me curious about ocean heat content in the tropical Pacific, how it has changed, and especially what geographic pattern it shows. So, I retrieved ocean heat content data for the upper 700m of the ocean (from NODC, via Climate Explorer).

Here’s the data for the tropical Pacific, which I defined as latitudes from 20S to 20N, longitudes from 120E to 280E:

There’s a clear upward trend overall, but just as clearly the increase has been highly irregular. What this doesn’t tell us, is anything about the contrast between ocean heat content in the western and eastern tropical Pacific regions.

I decided to look at OHC data in small longitude bands, 10 degrees wide each (and extending from 20S to 20N latitude), from longitude 120 to 280 (so the first band is 120-130E, the next 130-140E, etc.). This amounts to 16 longitude bands covering the tropical Pacific. In order to find which patterns dominate these time series, I applied PCA (principal component analysis) [technical note: because all the time series are the same variable on the same scale, the time series were centered but not normalized before computing PCA].

The first PC (principal component) dominates, accounting for 67% of the variance of the data. Usually when such is the case, the dominant PC is close to the overall average of the data (which would well approximate the overall trend). But in this case, the dominant PC turns out to be the contrast between west and east Pacific. Here are the “loadings” (or what I call the loadings, the weight assigned to each longitude band) as well as the resultant time series:

The graph on the left clearly shows that this is the difference between western and eastern Pacific data. When that on the right (the time series) is positive, the west has more heat content than the east, which roughly corresponds to la Nina-like conditions, but when it’s negative it roughly corresponds to el Nino-like conditions. Note particularly the big dip around 1998 when we experienced the very strong el Nino. Also obvious is that the trend over the most recent decade or so has been decidedly upward (more heat in the west, less in the east) which confirms that this pattern anti-correlates with surface air temperature time series.

England et al. also demonstrate the very recent trend in surface winds over the tropical Pacific which is the cause of much of the fluctuations in ocean heat content, especially the contrast between east and west. This lends considerable credence to their hypothesis, and the fact that it is in accord with the ideas of others makes it yet more credible.

Perhaps the million-dollar question is: what will happen when the enhanced winds subside (essentially, when we see another el Nino event)? The simulations by England et al. suggest that surface warming will resume, and that for the most part little or no trace will be left of the so-called “pause.” If that’s the case, we should expect record-high global average temperatures and an end to talk about the “pause” except from the most hard-core deniers.

But some (e.g. Mike Mann) suspect that the dominance of la Nina-like conditions recently may actually be a result of climate change. If so, it may be quite a while before we see anything like a strong (or even moderate) el Nino. We can, however, expect warming to continue even in a persistently non-el Nino world, a la this post by John Nielsen-Gammon.

20 responses to “Gone with the Wind”

One of the first things I would like to know is what is behind the may last until 2020/change in 2020 logic. Is there physics-based explanation for that, or is it just a guess based upon past behavior?

I find it helpful to think of the wind dragging rather than pushing the water.
The currents that are set up (gyres) then may be said to push water deeper.
And upwelling may be thought of as replacing surface water that is dragged away.
But that’s just what works best for me.

“It takes a century of global warming to reach the amplitude of the change in circulation associated with a flip from El Niño to La Niña. This might seem small, until you think about the implications of a shift in the mean comparable to the peak-to-peak variations in the ENSO cycle. Well before this point one would reach a situation in which the shift in the mean is comparable to changes in circulation or rainfall averaged over one or two decades. Whether we are already approaching the latter point is obviously a key question in climate research….”

I was looking, somewhere there he points out that when the winds change, they push the surface water — and that it’s hard to tell whether seeing warm water deeper in the ocean at any particular point represents a temporary ‘slosh’ of the warm water to that side of the ocean, or represents mixing of warm water deeper into the ocean there. The answer comes when the wind changes — if it’s the surface layer being pushed one way, then it gets pushed the other way and the warm water is now going to be deeper on the other side of the ocean. But if the warm water got mixed into the deeper water, it stays there when the wind changes.

But some (e.g. Mike Mann) suspect that the dominance of la Nina-like conditions recently may actually be a result of climate change.

This has been concerning me for quite a while.

If the warming has resulted in a new metastability that pumps more heat under the ocean surface than would have occurred had no ENSO cycles remained as they were in the 20th century, then the damping down of the rate of surface warming that would otherwise have occurred might exacerbate the reluctance of governments and business of this generation to act to mitigate carbon emissions.

The long-term consequences would then likely be worse than currently modeled, both from a shift toward the higher end of the possible emissions alternatives, and because the future thermal momentum of the global system would necessarily be higher, both from the exacerbated emissions and from the character of the structuring of the global system that will eventually ‘slosh’ the other way.

If cooler, sooner for current generations means hotter, longer than is projected for future generations, then the consequences for future life and economies with be far more grim. And you can take that to the bank.

I’m a bit confused by England et al’s figure 5 (also shown in the realclimate post, under the schematic) – it seems to indicate that recent observations are well below the range of IPCC projections. This does not seem consistent with the view that current trends are more or less in line with projections, as shown by IPCC AR5 Figure 1.4 (eg https://www.skepticalscience.com/curry-mcintyre-resist-ipcc-model-accuracy.html). Is this a case of different baselines, or different models?

An old denier trick is to take the statement, “The IPCC predicted a rise of at least ~.2 degrees per decade” (at least in some scenarios, but I won’t go there) and then tack that on to the local max observed in 1990. While the trend is ~.16 (GISS) is, in fact, lower than .2, by tacking the “expected trend” on the 1990 local max, that exacerbates the apparent difference.

To those who don’t know enough to know that a line has 2 parameters, an intercept and a slope, this can be an effective way to contribute additional FUD.

This is the same “trick” used to “show” that there has been “no warming since 1998”.

The East Pacific is cool because of cool water upwelling from the depths, which gets blown by the easterlies over to the west side of the Pacific. The west is warm because the water there has been sitting on the surface soaking up tropical sun from the time it reached the surface in the east. Global warming affects the surface waters faster than the deep, so it seems reasonable the west would warm faster than the east. This increases the easterly winds and brings up more cold water from the depths, so this could be a negative feedback that some contrarians have been hoping for.

But if a shift to La Nina conditions reduces the warming to a lower rate, is it good or bad news? More frequent and/or more severe La Ninas could have significant impacts on regional climate, droughts/floods etc. Was the severe weather in the very strong 10/11 La Nina primarily a natural ENSO event (eg massive Australian floods), or did CO2 play a hand more significant than just loading the atmosphere with more water vapour and energy? And then when we do get an El Nino year (as happened in 09/10) this cooling effect dissapears.

And how much worse can it get? It may be easy to shift the Walker Circulation from El Nino (no easterlies, or even westerlies) or neutral (moderate easterlies) into La Nina (strong easterlies), but it may be much harder to significantly strengthen the easterlies beyond what we see in a currently strong La Nina. In this case the cooling from this negative feedback would be limited to the current warming trend for La Nina years, which is warming as fast as the El Nino years, but from a lower baseline. Or perhaps it will create a new state with stronger La NInas that could push the global temperature trend below the current trend line for temps in La NIna years. If this happened I would expect frequent La Nina events significantly more severe than what we saw in 10/11.

So the contrarians would get to say ‘see I told you the warming wasn’t as bad as predicted. Only some of the years (the El Nino ones) are that warm. And yes we see much more La NIna associated severe weather, but La NIna is natural.’

I have to learn more about this deep heat content increase. Presumably true irreversible heat transfer could only occur to this depth on this time scale in deep or intermediate water formation regions in the subpolar oceans. But the heat content below some depth can also change simply due to the deformation of constant density surfaces due to the wind or more generally as an adjustment to what is happening to the density field above this level. This is completely and rapidly reversible when these density surfaces return to a less deformed shape. Oceanographers try to remove this effect to focus better on the more irreversible component of the heat redistribution by doing a census of waters with different densities. Is the mass of water between specified density values changing systematically or is it just that the boundary between different density layers is deformed adiabatically? The latter would have little relevance for the surface. I haven’t seen this kind of analysis for these recent observations.

Your intuition would be right, I think, if this heat content change were fairly uniform over these deep layers and were due to true mixing or transport and not to adiabatic sloshing. In post #8, I refer to this effect of deep ocean warming on the surface at a later date as the recalcitrant component of global warming.”

1) The ‘loadings’ you speak of are determined by the analytic process;
2) Their values indicate the relative contribution of each band to the variance pattern of the PC to which they belong;
3) The ‘relative contribution’ is related to heat content, not pure statistical ‘weight’–that is, I’m taking it that the low values for the Easternmost bands indicate relative coolness, not relative insignificance in contributing to the pattern of the PC.

[Response: The negative values for the easternmost bands do indeed indicate relative coolness *when the PC time series is high* and relative warmth *when the PC time series is low*.]

As you can probably tell, despite hanging around here for some years now (and going with some care through Dr. Mann’s nice explanation of PC analysis in “Hockey Stick”), I know just slightly more about PC analysis than my dog knows about algebra.

I’m glad I’m not alone in struggling with understanding PCA. This looks like a really neat application of it, though, and I think it illustrates very nicely what England et al and others have been saying about what is going on in the tropical Pacific.

One thought: The actual amount of sea available is both much shallower and much smaller in your highest longitudes due to the presence of shallows and land there. I note they contribute quite a lot less to the first principal component, as well. Wonder if there might be some adjustment that might need to be made for that in any deeper analysis of this sort?

[Response: Can’t say for sure off the top of my head, but the data are GJ/m^2 rather than GJ, which compensates fot the smaller area.]

One question: The 1st PC is really 67% of all the variance? These loadings as a whole seem a bit low for that (without running the stats myself).

[Response: Yes, it really is. The loadings vector is normalized, so it’s dimensionless.]

I’m curious about how far we can go with pinning temperature variations to physical phenomena and model it at ever shorter scale and how much internal variability will remain ‘random’ or unpredictable. I’m not sure even what terminology is correct but… is there a boundary for random internal variation? Will climate science and meteorology – already sharing much -eventually meet in the middle and fill that ground between?

What is not random is the quantifiable total heat content, energy imbalance and the magnitude of the solid and fluid thermal buffers available to absorb that heat. Although the path to the final result may appear to be locally Brownian, the trend to the final result is clear, the physical basis of that trend and final result are indisputable, and the final result is inevitable.

It’s interesting to look at the Southern Oscillation Index, which measures the difference in air pressure between Darwin, Australia, and Tahiti. With positive values indicating strong trades (La Niña conditions), the five year average SOI for 2011, 2012, and 2013 were the highest, third highest, and fourth highest on record since 1880. Outside of the current decade, only 1975 cracks the top four spots.

Apparently Michael Ventrice, operational scientist for the Energy team at Weather Services International, believes there could be a strong El Niño event beginning this year if the westerly Pacific wind bursts keep up.